I'm not gonna pretend I know a ton about nitty gritty balancing, but I love the concepts you have! Any hero who gets stacks/gives stacks to enemies is super interesting to me, I'd probably love playing as them. And your story writing is impeccable! I can already map out interactions in my head...
way too much of a must-buy imo, this would be an easy counter to hero kits like dynamo, seven, or rem that doesn't require any skill like counterspell does.
I could see a small issue with it upgrading into counterspell, usually upgraded items don't lose stats while rebuttal would lose a good amount if it upgraded into counterspell. Grudge seems like a pretty good item concept though!